Before I start, I just want to say that I am usually a person who is the first to criticize any series which are popular among the majority. In addition, I am the person who has never experienced that feeling you are just going to cry due to some movements or sounds coming from the TV series/movies which want to cause emotions to you. But this k-drama... DAMN... Up to now, I considered Game of thrones (seasons 2-5) and Mr. Sunshine objectively as the biggest masterpiece when it comes to series, but I guess, this k-drama totally overtook this position.

Firstly, all hail to the actors. I think, I have never been so hooked up into literally ALL of the characters. Each one of them (leads, seconds and supports) did an excellent job. I always take care of not doing any sounds when everyone around me sleeps and I am watching dramas, but they made me laugh as hell even at 2AM, made me cry (it is true that only for a few seconds, but I have not cried for 15 years) multiple times and made me totally fangirling over them.
Secondly, the story was really well written. Not even a single scene was a waste of time (what, in my opinion occurs quite often in other series), the story let you nervous almost whole the time and you never know what will happen next. The only one thing which was not perfect was the totally last scene (about the last one or two minutes). I think, the end without that scene would be much more satisfying in my opinion.
Lastly, I have never really been into re-watching any series (expect a few favourite moments), but I am definitely for watching this k-drama one day again.

And here, there is some final sum of the points.
Actors: 20 Hyun Bins from 10
Story: 9.99999999 flying tractors from 10 (sorry last scene)
Music: 10 downloaded albums from internet cafés from 10
Would I go through this emotional hell again? Definitely YES!
Overall score: 85648356187451 bugs from 100